1. Field of the Invention
The present invention generally relates to a wire bonding method and a chip structure. More particularly, the present invention relates to a wire bonding method and a chip structure using the same.
2. Description of Related Art
Modern electronic equipment relies heavily on printed circuit boards on which semiconductor chips, or integrated circuits (ICs), are mounted. The mechanical and electrical connections between the chip and the substrate have posed challenges for chip designers. Wire bonding is one of the well known techniques for interconnecting the IC to the substrate.
In general, wire bonding technique includes “forward bonding” and “reversed bonding”. Forward bonding refers to bonding from an integrated circuit/component down to a substrate. A forward bonding process places a ball bond on the die first, a capillary then forms a stitch bond from the ball bond to a pad of the substrate. Most wire bonding applications use the typical forward bonding process, because it is faster and more capable of finer pitch than reverse bonding. However, forward bonding has a loop height constraint due to the neck area above the ball. Excessive bending above the ball can cause neck cracks, which results in reliability problems. Therefore, the chip structure using forward bonding process is hard to meet the requirements of low-profile looping.
A reverse bonding process or so called Stitch-bond Stands on Bump (SSB), on the contrary, places a bump on the die pad first. After the bump is formed, a ball bond is placed on the substrate, followed by a stitch bond on the bump. With this metal ball bump on the pad of chip, it can create buffer to keep the high down force of stitch bond from damaging the underlayer structures beneath the pad. Low-profile looping requirements have propelled the growing use of reverse ball bonding. However, reverse bonding is a discontinuous process which is a much slower process than forward bonding. Therefore, how to meet the low-profile looping requirements but also keep efficient in manufacture is an important research topic in current industry.